Willis Lamm's
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Traffic Signal Lenses |
Many early traffic signals designs were based on railroad signal technology however early on traffic signal designers discovered a significant difference between railroad and highway signals. Railroad signals were aimed down a specific and somewhat narrow track. They needed to project a fairly weak light source a long distance in a specific direction. Traffic signals had to be easily visible from a much wider angle by traffic and pedestrians. Therefore the Fresnel lenses used by the railroads were found to be inadequate for highway use.
A number of ideas and patents evolved that projected light straight out, but also down and to the sides. Frosted and "orange peel" lenses tended to cause "phantoms" (appear to be lit when not lit) when struck by low angle sunlight. Manufacturers experimented with a variety of lens, reflector and visor patterns. Lenses evolved into the "small bead" lenses used in modern day signals. The beads on the inside of the lenses are typically made relatively flat on the bottom to trap light from the reflector but to minimize reflected sunlight. Therefore it was critical that lenses be installed correctly and most of them have "TOP" cast in tiny letters at the top of the lens, lest a signal maintenance person might have a "senior moment" while replacing a lens. Early lenses also came in lettered (command) and non-lettered configurations. Early signals were not standardized and it was a while until all areas of the country had electric traffic signals. Therefore many communities opted to have "commands" displayed on signal lenses to minimize confusion by unfamiliar drivers. This section illustrates a variety of different lens types and their applications. |
Adler Bar Lenses
In 1934 inventor Charles Adler, Jr. patented a series of traffic lenses designed for the colorblind. These lenses displayed bars that clarified the signal's indication. A vertical bar indicated "go." A diagonal bar indicated "caution." A horizontal bar indicated "stop." As early drivers became more familiar with traffic signals the need for lettered and bar lenses diminished, however a small number can still be seen in older signals in some communities. Here is a more complete feature on the Adler Bar Lens. |